Add **kwargs to is_valid and several validation methods

[jorgeecardona]

I like to use forms a lot, but sometimes I have to overwrite __init__ to accept some parameters that change the behaviour of the validation, I was thinking that maybe is better just to add a way to pass some data to the validation methods of the form in order to validate it.

Imagine that I want to create a form to change the email of an user and I don't want to use an argument in the request for "username" and use the username from the session framework (I just want to avoid to send the username in the request), I will have to overwrite the __init__ of the form in order to accept "user" as an argument and store it in self.user to be used later in clean_email. Instead, if we have this new argument in is_valid is just to do: form.is_valid(user=user).

This new argument doesn't hurt anyone, and It will help a lot to some of us that use form in this way.